Hydrogen Storage Alloy, Hydrogen Storage Alloy Electrode, Secondary Battery, And Method For Producing Hydrogen Storage Alloy

ABSTRACT

Provided is a hydrogen storage alloy which is characterized in that two or more crystal phases having different crystal structures are layered in a c-axis direction of the crystal structures. The hydrogen storage alloy is further characterized in that a difference between a maximum value and a minimum value of a lattice constant a in the crystal structures of the laminated two or more crystal phases is 0.03 Å or less.

TECHNICAL FIELD

The invention relates to a hydrogen storage alloy, a hydrogen storagealloy electrode, a secondary battery, and a method for producing ahydrogen storage alloy.

BACKGROUND ART

A hydrogen storage alloy is an alloy capable of safely and easilystoring hydrogen as an energy source and has drawn an attention as a newenergy conversion and storage material and its application fields are ina wide range, e.g., hydrogen storage and transportation, heat storageand transportation, heat-mechanical energy conversion, separation andpurification of hydrogen, isolation of hydrogen isotopes, batteriesusing hydrogen as active materials, catalysts for synthetic chemical,temperature sensors, and so forth.

For example, a nickel-metal hydride battery using a hydrogen storagealloy as a negative electrode material has characteristics such as (a)high capacity; (b) durability to overcharge and over discharge; (c)capability of charging and discharging at high efficiency; (d) cleannessand has been actively investigated to have further improved capabilities(improvement of retention ratio of capacity in the case of repeatingcharge and discharge, that is, cycle life, improved capacity of thebattery, etc.).

So far, an AB₅ type rare earth-Ni-based alloy having a CaCu₅ typecrystal structure has been put into practical use as an electrodematerial for a nickel-metal hydride battery, one application example ofsuch a hydrogen storage alloy; however the discharge capacity reachesalmost a limit of about 300 mAh/g and presently it becomes difficult tofurther improve the capacity.

Further, as a new hydrogen storage alloy, a rare earth metal-Mg—Ni basedalloy, for example, LaCaMgNi₉ alloys (Patent Document 1) having a PuNi₃type crystal structure have drawn attention and it is reported that adischarge capacity exceeding that of an AB₅ type alloy can be obtainedby using these alloy for electrode materials.

It is also reported that in addition of the crystal phase having the AB₅type crystal structure, electrode materials of hydrogen storage alloyscontaining a crystal phase of AB₂ type crystal structure such as MgCu₂type or rare earth metal-Mg—Ni type alloys containing, as a main phase,the crystal phase having Ce₂Ni₇ type, CeNi₃ type, or Gd₂Co₇ type crystalstructure have high hydrogen storage capacities and show good hydrogenrelease characteristics (Patent Document 2).

Furthermore, with respect to alloys having Ce₅CO₁₉ type crystalstructure, it is reported that electrodes complexed with rare earth-Nialloy having a CaCu₅ type crystal structure are excellent inhydrogenation reaction speed (Patent Document 3).

Patent Document 1: Japanese Patent No. 3015885

Patent Document 2: Japanese Patent Application Laid-Open (JP-A) No.11-323469

Patent Document 3: Japanese Patent No. 3490871

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

However, in the above-mentioned conventional hydrogen storage alloys, aproblem that the hydrogen storage capacity is decreased in the casewhere hydrogen storage and release are repeated has not sufficientlybeen solved yet.

Further, with respect to the conventional hydrogen storage alloys, thereis another problem that when the hydrogen storage alloys are made to beexcellent in the cycle life to quickly release the stored hydrogen, thestored hydrogen is gradually released simply by leaving the hydrogenstorage alloys as they are.

In view of the above state of the art, one aim of the invention is toprovide a hydrogen storage alloy of which the hydrogen storage capacityis hardly decreased even if hydrogen storage and release are repeated,that is, a hydrogen storage alloy excellent in the cycle life.

Another aim of the invention is to provide a hydrogen storage alloyexcellent in the cycle life and having high hydrogen storage amount.

Further, another aim of the invention is to provide a hydrogen storagealloy having little self-release of hydrogen while maintaining theexcellent cycle life.

Means for Solving the Problems

To solve the above-mentioned problems, the inventors of the inventionhave made various investigations and accordingly have found that ahydrogen storage alloy having a layered structure of a plurality ofcrystal phases with different crystal structures such as a crystal phaseof Pr₅Co₁₉ type crystal structure, a crystal phase of Ce₂Ni₇ typecrystal structure, and the like can exhibit remarkably excellent cyclelife and the finding now leads to completion of the invention.

That is, the invention provides the following hydrogen storage alloy, ahydrogen storage alloy electrode containing the hydrogen storage alloy,a secondary battery comprising the electrode, and a method of producingthe hydrogen storage alloy.

(1) A hydrogen storage alloy containing two or more crystal phaseshaving different crystal structures which are layered in the c-axisdirection of the crystal structures.

(2) The hydrogen storage alloy according to the above description (1) inwhich the difference of the maximum value and the minimum value of thelattice constant a in the crystal structures of the layered two or morecrystal phases is 0.03 Å or less.

(3) The hydrogen storage alloy according to the above description (1) or(2) in which the crystal phases include two or more types selected froma group consisting of a crystal phase having La₅MgNi₂₄ type crystalstructure, a crystal phase having Ce₅Co₁₉ type crystal structure, and acrystal phase having Ce₂Ni₇ type crystal structure.

(4) The hydrogen storage alloy according to one of the abovedescriptions (1) to (3) having a composition defined by a generalformula Ra1_(a)R2_(b)R3_(c) (wherein R1 is one or more kind elementsselected from a group consisting of rare earth metals including Y; R2 isone or more kind elements selected from a group consisting of Mg, Ca,Sr, and Ba; R3 is one or more kind elements selected from a groupconsisting of Ni, Co, Mn, Al, Cr, Fe, Cu, Zn, Si, Sn, V, Nb, Ta, Ti, Zr,and Hf; and a, b, and c satisfy 10≦a≦30; 1≦b ≦10; 65≦c≦90; anda+b+c=100).

(5) The hydrogen storage alloy according to the above description (1)having a composition defined by a general formulaR1_(d)R2_(e)R4_(f)R5_(g) (wherein R1 is one or more kind elementsselected from a group consisting of rare earth metals including Y; R2 isone or more kind elements selected from a group consisting of Mg, Ca,Sr, and Ba; R4 is one or more kind elements selected from a groupconsisting of Ni, Co, Cr, Fe, Cu, Zn, Si, Sn, V, Nb, Ta, Zr, and Hf; R5is one or two elements selected from Mn and Al; and d, e, f, and gsatisfy 8≦d≦19; 2≦e≦9; 73≦f≦79; 1≦g≦4; and d+e+f+g=100) and satisfying3.53≦(B/A)≦3.80 and 0.0593(B/A)+1.59≦rA≦0.0063(B/A)+1.81 in the case(B/A) is defined as (f+g)/(d+e) and rA (A) is defined as the averageatom radius of the R1 and R2.

(6) The hydrogen storage alloy according to the above description (5) inwhich R1 consists of one or more kind elements R1′ selected from a groupconsisting of Ce, Pr, Nd, Sm, and Y and La and the ratio of La/R1′ is 5or less; the R2 is Mg; the R4 is one or two elements selected from Niand Co; the R5 is Al; and d, e, f, and g satisfy 16≦d≦19; 2≦e≦5;73≦f≦78; and 2≦g≦4.

(7) The hydrogen storage alloy according to the above description (5) or(6) having a main generative phase is a crystal phase having Pr₅Co₁₉type crystal structure or a crystal phase having Ce₅Co₁₉ type crystalstructure.

(8) The hydrogen storage alloy according to the above description (1)having, as a main generative phase, a crystal phase having Ce₅Co₁₉ typecrystal structure and a composition defined by a general formulaLa_(h)R6_(i)R7_(j)Mg_(k)R8_(m) (wherein R6 is one or more kind elementsselected from a group consisting of rare earth metals including Y andexcluding La; R7 is one or more kind elements selected from a groupconsisting of Zr, Ti, Zn, Sn and V; R8 is one or more kind elementsselected from a group consisting of Ni, Co, Mn, Al, Cu, Fe, Cr, and Si;and h, i, j, k and m satisfy 0≦j≦0.65; 2≦k≦5.5; 0.70≦h/(h+i)≦0.85; andh+i+j+k+m=100).

(9) The hydrogen storage alloy according to the above description (1) inwhich the ratio of the crystal phase having CaCu₅ type crystal structureis 22% by weight or less.

(10) The hydrogen storage alloy according to the above description (9)in which the hydrogen equilibrium pressure is 0.07 MPa or less.

(11) The hydrogen storage alloy according to the above description (9)or (10) having a composition defined by a general formulaR1_(n)R2_(p)R4_(q)R5_(r) (wherein R1 is one or more kind elementsselected from a group consisting of rare earth metals including Y; R2 isone or more kind elements selected from a group consisting of Mg, Ca,Sr, and Ba; R4 is one or more kind elements selected from a groupconsisting of Ni, Co, Cr, Fe, Cu, Zn, Si, Sn, V, Nb, Ta, Ti, Zr, and HfR5 is one or two kind elements selected from Mn and Al; and n, p, q, andr satisfy 16≦n≦23; 2≦p≦8; 68.5≦q≦76; 1≦r≦6.5; and n+p+q+r=100).

(12) The hydrogen storage alloy according to one of the abovedescriptions (9) to (11) wherein the content of Mn is 5% by weight orless.

(13) A hydrogen storage alloy electrode using the hydrogen storage alloyaccording to any one of the above descriptions (1) to (12) as a hydrogenstorage medium.

(14) A secondary battery using the hydrogen storage alloy electrodeaccording to the above description (13) as a negative electrode.

(15) A method for producing the hydrogen storage alloy according to oneof the above descriptions (1) to (12), comprising a melting step of heatmelting alloy raw materials at prescribed mixing ratio in inert gasatmosphere; a cooling step of rapid solidification the melted alloy at acooling speed of 1000° C./s or higher; and an annealing step of furtherannealing the alloy subjected to the cooling step at 860° C. or higherand 1000° C. or lower in inert gas atmosphere in pressurized state.

Herein, crystal phase in the invention means a region where a singlecrystal structure exists.

Effect of the Invention

With respect to conventional hydrogen storage alloys, some contain twoor more crystal phases having crystal structures different from oneanother. However, unlike the invention, these crystal phases are notlayered in the c-axis direction and exist independently in individualregions. Therefore, it is supposed that significant lattice strains arecaused in the respective crystal phases at the time of absorption andrelease of hydrogen are repeated and as a result, deterioration of thealloys such as pulverization is caused when absorption and release ofhydrogen are repeated, resulting in aggravation of the cycle life.

On the other hand, with respect to the hydrogen storage alloy of theinvention, two or more crystal phases having crystal structuresdifferent from one another are layered in the c-axis direction of thecrystal structures. Therefore, the strains of the crystal phases causedbecause of repeated absorption and release of hydrogen are remarkablymoderated. Such moderation of the strains suppresses deterioration atthe time of repeating absorption and release of hydrogen and as aresult, the cycle life can remarkably be improved.

Accordingly, the hydrogen storage alloy of the invention is providedwith an effect to cause an excellent cycle life. Further, since thehydrogen storage alloy electrode and the secondary battery of theinvention are configured by using such a hydrogen storage alloy, even ifdischarge and charge are repeated, they are provided with an excellentproperty that the discharge capacity is hardly decreased. Furthermore,the method for producing the hydrogen storage alloy of the invention isprovided with an effect to efficiently produce such a hydrogen storagealloy.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1: A schematic drawing showing one embodiment of a first hydrogenstorage alloy.

FIG. 2: A TEM image showing one example of the first hydrogen storagealloy.

FIG. 3: A magnified drawing of a portion of FIG. 2.

FIG. 4: A photograph showing the distribution of Ni and Mg obtained byEPMA, with respect to Example 1 and Comparative Example 1.

FIG. 5: A graph showing the measurement results of cycle life, withrespect to hydrogen storage alloys of Example 1 and Comparative Example1.

FIG. 6: A graph showing a relation of the difference of the a-axislength and the capacity retention ratio, with respect to the hydrogenstorage alloys of Examples 2 and 6.

FIG. 7: A graph formed by plotting the evaluation results of the cyclelife and discharge capacity using B/A and rA (A) as coordinate axes,with respect to hydrogen storage alloys of Examples 7 to 42.

FIG. 8: A graph showing the relation of the capacity retention ratio toCaCu₅ phase production ratio, with respect to hydrogen storage alloys ofExamples 82 to 91.

FIG. 9: A graph showing the relation of the remaining discharge capacityto hydrogen equilibrium pressure, with respect to hydrogen storagealloys of Examples 92 to 101.

FIG. 10: A graph formed by plotting the Ce content and the cycle life,with respect to hydrogen storage alloys of Examples 102 to 109.

BEST MODES OF THE EMBODIMENTS (First Hydrogen Storage Alloy)

The first hydrogen storage alloy of the invention contains two or morecrystal phases having crystal structures different from one another andlayered in the c-axis direction of the crystal structures.

Examples of the above-mentioned crystal phases are a crystal phasehaving a rhombohedral La₅MgNi₂₄ type crystal structure (hereinafter,sometimes referred to simply La₅MgNi₂₄ phase), a crystal phase having ahexagonal Pr₅Co₁₉ type crystal structure (hereinafter, sometimesreferred to simply Pr₅Co₁₉ phase), a crystal phase having a rhombohedralCe₅Co₁₉ type crystal structure (hereinafter, sometimes referred tosimply Ce₅Co₁₉ phase), a crystal phase having a hexagonal Ce₂Ni₇ typecrystal structure (hereinafter, sometimes referred to simply Ce₂Ni₇phase), a crystal phase having a rhombohedral Gd₂Co₇ type crystalstructure (hereinafter, sometimes referred to simply Gd₂Co₇ phase), acrystal phase having a hexagonal CaCu₅ type crystal structure(hereinafter, sometimes referred to simply CaCu₅ phase), a crystal phasehaving a cubic AuBe₅ type crystal structure (hereinafter, sometimesreferred to simply AuBe₅ type phase), and a crystal phase having arhombohedral PuNi₃ type crystal structure (hereinafter, sometimesreferred to simply PuNi₃ phase).

Particularly, it is preferable to layer two or more kind crystal phasesselected from a group consisting of La₅MgNi₂₄ phase, Pr₅Co₁₉ phase,Ce₅Co₁₉ phase, and Ce₂Ni₇ phase. A hydrogen storage alloy having layeredstructure of these crystal phases cause less strains since thedifference of the expansion and shrinkage ratio among the respectivecrystal phases and thus has an excellent property that lessdeterioration is caused at the time of repeating hydrogen absorption andrelease.

Herein, the La₅MgNi₂₄ type crystal structure means a crystal structureformed by inserting 4 AB₅ units between A₂B₄ units: the Pr₅Co₁₉ typecrystal structure means a crystal structure formed by inserting 3 AB₅units between A₂B₄ units: the Ce₅Co_(is) type crystal structure means acrystal structure formed by inserting 3 AB₅ units between A₂B₄ units:the Ce₂Ni₇ type crystal structure means a crystal structure formed byinserting 2 AB₅ units between A₂B₄ units: the Gd₂Co₇ type crystalstructure means a crystal structure formed by inserting 2 AB₅ unitsbetween A₂B₄ units: and the AuBe₅ type crystal structure means a crystalstructure composed of A₂B₄ units alone.

In addition, the A₂B₄ unit means a crystal lattice having a hexagonalMgZn₂ type crystal structure (C14 structure) or a hexagonal MgCu₂ typecrystal structure (C15 structure) and AB₅ unit means a crystal latticehaving a hexagonal CaCu₅ type crystal structure.

Further, A denotes any element selected from a group consisting of rareearth metal elements and Mg and B denotes any element selected from agroup consisting of transition metal elements and A1.

The layering order of the above-mentioned respective crystal phases isnot particularly limited and any specified crystal phase combination maybe layered repeatedly with periodicity or the respective crystal phasesmay be layered at random without periodicity.

In addition, with respect to the crystal phases having theabove-mentioned respective crystal structures, the crystal structurescan be specified by carrying out X-ray diffraction for milled alloypowders and analyzing the obtained X-ray diffraction patterns byRietveld method.

A schematic drawing of one embodiment of the first hydrogen storagealloy is shown in FIG. 1. As shown in FIG. 1, one embodiment of thefirst hydrogen storage alloy is configured by layering the CaCu₅ phase,two Pr₅Co₁₉ phases neighboring to the CaCu₅ phase, and two Ce₂Ni₇ phaseneighboring to the Pr₅Co₁₉ phases in the c-axis direction of the crystalstructure.

Observation of the lattice image of the alloy by TEM makes it possibleto confirm the fact that two or more crystal phases having differentcrystal structures are layered in the c-axis direction of the crystalstructures One example of the lattice image of the first hydrogenstorage alloy of the invention is shown in FIG. 2 and FIG. 3.

These drawings shows that this hydrogen storage alloy is configured bylayering a crystal phase formed by repeating arrangement of 3 AB₅ unitsinserted between A₂B₄ units and a crystal phase formed by repeatingarrangement of 4 AB₅ units inserted between A₂B₄ units in the c-axisdirection. The former crystal phase is a crystal phase having theCe₅Co₁₉ phase crystal structure and the latter is a crystal phase havingthe LaMgNi₂₄ type crystal structure. Observation of the lattice image byTEM in such a manner makes it possible to confirm the fact that two ormore crystal phases having different crystal structures are layered inthe c-axis direction, which is a constituent factor of the invention.

As described, since the first hydrogen storage alloy of the inventioncomprises two or more crystal phases having different crystal structuresand layered in the c-axis direction, the strains of the crystal phasesat the time of absorbing hydrogen can be moderated by neighboring othercrystal phases. Accordingly, even if hydrogen absorption and release arerepeated, less pulverization of the alloy is caused and thus anexcellent cycle life can be caused.

(Second Hydrogen Storage Alloy)

The second hydrogen storage alloy of the invention is configured byadjusting the difference of the maximum value and the minimum value ofthe lattice constant a (hereinafter, also referred to as a-axis length)in the crystal structures of the layered two or more crystal phases tobe 0.03 Å or less in the first hydrogen storage alloy.

When the difference of the maximum value and the minimum value of thea-axis length of the respective crystal phases is configured by adjustedto be 0.03 Å or less, the strains among the respective crystal phasescaused at the time of hydrogen absorption and release are furtherreduced and the hydrogen storage alloy becomes difficult to bepulverized even if hydrogen absorption and release are repeated, thatis, the hydrogen storage alloy becomes excellent in the cycle life.

The difference of the maximum value and the minimum value of the a-axislength of the respective crystal phases is preferably 0.02 Å or less,more preferably 0.016 Å or less, and even more preferably 0.01 Å orless.

When the difference of the maximum value and the minimum value of thea-axis length is in the above-mentioned range, the capacity retentionratio of the hydrogen storage alloy is further improved and the cyclelife can be improved.

Herein, the a-axis length in the invention can be measured by carryingout crystal structure analysis of the hydrogen storage alloy by an X-raydiffraction apparatus. More practically, the a-axis length can becalculated for each crystal phase by determining the lattice constant ofeach crystal phase from XRD patterns by Rietveld method (analysissoftware: RIETAN2000).

The above-mentioned first or second hydrogen storage alloy is preferableto have a composition defined by a general formula R1_(a)R2_(b)R3_(c)(wherein R1 is one or more kind elements selected from a groupconsisting of rare earth metals including Y; R2 is one or more kindelements selected from a group consisting of Mg, Ca, Sr, and Ba; R3 isone or more kind elements selected from a group consisting of Ni, Co,Mn, Al, Cr, Fe, Cu, Zn, Si, Sn, V, Nb, Ta, Ti, Zr, and Hf; and a, b, andc satisfy 10≦a≦30; 1≦b≦10; 65≦c≦90; and a+b+c=100).

(Third Hydrogen Storage Alloy)

The third hydrogen storage alloy of the invention is the first hydrogenstorage alloy further having a composition defined by a general formulaR1_(d)R2_(e)R4_(f)R5_(g) (wherein R1 is one or more kind elementsselected from a group consisting of rare earth metals including Y; R2 isone or more kind elements selected from a group consisting of Mg, Ca,Sr, and Ba; R4 is one or more kind elements selected from a groupconsisting of Ni, Co, Cr, Fe, Cu, Zn, Si, Sn, V, Nb, Ta, Ti, Zr, and Hf;R5 is one or two elements selected from Mn and Al; and d, e, f, and gsatisfy 8≦d≦19; 2≦e≦9; 73≦f≦79; 1≦g≦4; and d+e+f+g=100) and satisfying3.53≦(B/A)≦3.80 and 0.0593(B/A)+1.59≦rA≦0.0063(B/A)+1.81, preferably0.0593(B/A)+1.59≦rA≦1.827, in the case (B/A) is defined as (f+g)/(d+e)and rA (Å) is defined as the average atomic radius of R1 and R2.

In the case the average atom radius rA (Å) of the R1 and R2 elements(that is, the element in the A side) composing the crystal structures ofthe hydrogen storage alloy and the ratio (B/A) of the R1 and R2 elements(that is, the elements in the A side) to the R4 and R5 elements (thatis, the elements in the B side) satisfy the following relationalexpressions: 3.53≦(B/A)≦3.80 and 0.0593(B/A)+1.59≦rA≦0.0063(B/A)+1.81:the R2 element tends to be included in the A₂B₄ units and as a result,segregation of the R2 element is prevented and it becomes easy to formthe layered body of the crystal phases having desired crystal structuresand accordingly, a hydrogen storage alloy excellent in the cycle lifecan be obtained.

The third hydrogen storage alloy, preferably, wherein the R1 consists ofone or more kind elements R1′ selected from a group consisting of Ce,Pr, Nd, Sm, and Y and La at La/R1′ ratio or 5 or less; the R2 is Mg; R4is one or two elements selected from Ni and Co; R5 is Al.

In the case where La is substituted with one or more kind elements R1′selected from a group consisting of Ce, Pr, Nd, Sm, and Y whose atomicradius are smaller than the La at La/R1′ ratio or 5 or less, and the R2is Mg, the R4 is one or two elements selected from Ni and Co, the R5 isA1, and the d, e, f, and g respectively satisfy 16≦d≦19, 2≦e≦5, 73≦f≦78,and 2≦g≦4; Mg as the R2 element tends to be included further easier inthe A₂B₄ units and accordingly, a hydrogen storage alloy excellent inthe cycle life can be obtained.

Third hydrogen storage alloy preferably contains a crystal phase havingPr₅Co₁₉ type crystal structure or a crystal phase having Ce₅Co₁₉ typecrystal structure and further preferably contains the crystal phase asthe main produced phase.

When the crystal phase having Pr₅Co₁₉ type crystal structure or thecrystal phase having Ce₅Co₁₉ type crystal structure is the main producedphase, the lattice expansion coefficient is small at the time ofhydrogen absorption and resulted in an action that less strains arecaused, thereby giving an effect to further improve the cycle life.

Herein, the main produced phase means the phase at the highestproduction ratio.

(Fourth Hydrogen Storage Alloy)

The fourth hydrogen storage alloy of the invention is configured tohave, in the said first hydrogen storage alloy, a crystal phase havingCe₅Co₁₉ type crystal structure and a composition defined by a generalformula La_(h)R6_(i)R7_(j)Mg_(k)R8_(m) (wherein R6 is one or more kindelements selected from a group consisting of rare earth metals includingY and excluding La; R7 is one or more kind elements selected from agroup consisting of Zr, Ti, Zn, Sn and V; R8 is one or more kindelements selected from a group consisting of Ni, Co, Mn, Al, Cu, Fe, Cr,and Si; and h, i, j, k and m satisfy 0.70≦h/(h+i)≦0.85; andh+i+j+k+m=100).

According to the fourth hydrogen storage alloy, since the crystal phasehaving Ce₅Co₁₉ type crystal structure is contained as an indispensablephase, the alloy becomes excellent in the cycle life and furthermore,since the ratio h/(h+i) of La to the total of La and R6 element is in arange satisfying 0.70≦h/(h+i)≦0.85, segregation of Mg can be preventedand the production ratio of the crystal phase having the CaCu₅ typecrystal structure which is inferior in the cycle life can be decreasedand on the other hand, the ratio of the crystal phase having the Ce₅Co₁₉type crystal structure which is excellent in the cycle life is increasedand as a result, a hydrogen storage alloy having high hydrogen storagecapacity and excellent in the cycle life can be obtained.

In the fourth hydrogen storage alloy, j is preferable to be 0 or higherand 0.65 or lower and more preferable to be 0.2 or higher and 0.65 orlower. When j is in the above-mentioned numeral range, because of theexistence of the R7 element (that is, one or more kind elements selectedfrom a group consisting of Zr, Ti, Zn, Sn and V), Mg becomes diff cultto segregate and the ratio of the crystal phase having the Ce₅Co₁₉ typecrystal structure is increased and the hydrogen storage capacity isincreased.

Further, in the fourth hydrogen storage alloy, k is preferable to be 2or higher and 5.5 or lower and more preferable to be 3 or higher and 5or lower. When k is in the above-mentioned numeral range, segregation ofMg is prevented and the hydrogen storage alloy is provided with highhydrogen storage capacity and becomes excellent in the cycle life.

(Fifth Hydrogen Storage Alloy)

The fifth hydrogen storage alloy of the invention is the hydrogenstorage alloy which contains 22% by weight or less of the crystal phasehaving the CaCu₅ type crystal structure according to the first hydrogenstorage alloy.

Conventionally, it is known that the crystal phase having the CaCu₅ typecrystal structure is excellent in the cycle life despite of a lowdischarge capacity. However, according to the results of theinvestigations which the inventors of the invention have made, it isfound that in a hydrogen storage alloy configured by layering two ormore layers of crystal phases having crystal structures different fromone another, if the CaCu₅ phase exists much, the cycle life contrarilybecomes difficult to be improved.

The fifth hydrogen storage alloy is made further excellent in the cyclelife by adjusting the CaCu₅ phase ratio to be 22% by weight or less.

Further, in the fifth hydrogen storage alloy, the hydrogen equilibriumpressure is 0.07 MPa or lower.

Conventionally, a hydrogen storage alloy has a property that hydrogenabsorption is difficult and release of absorbed hydrogen is easy in thecase of high hydrogen equilibrium pressure. Accordingly, if the highrate capability is improved for the hydrogen storage alloy, self-releaseof hydrogen becomes easy,

However, as a result of the investigations by the inventors, it is foundthat a good high rate capability can be obtained in the case where thehydrogen equilibrium pressure is set to be as low as 0.07 MPa or lowerin the hydrogen storage alloy comprising two or more layers of crystalphases having crystal structures different from one another andcontaining 22% by weight or less of the crystal phase having the CaCu₅type crystal structure. This seems due to improvement in diffusivity ofhydrogen in the alloy.

Accordingly, in the fifth hydrogen storage alloy, the high ratecapability is made excellent and self-release of hydrogen (in the caseof a battery, self discharge) becomes difficult by setting the hydrogenequilibrium pressure to be 0.07 MPa or lower.

Herein, the hydrogen equilibrium pressure means the equilibrium pressureat H/M=0.5 (equilibrium pressure in the case the ratio of hydrogen atomsto metal atoms is 0.5) in the PCT curve (pressure-composition isothermalline) at 80° C.

With respect to the fifth hydrogen storage alloy, the Mn content in thealloy is preferable to be 5% by weight or less. When the Mn content isadjusted to be 5% by weight or less, while the high rate capability ismaintained, the self-release of hydrogen can further be suppressed.

Further, the fifth hydrogen storage alloy is preferable to have acomposition defined by a general formula R1_(n)R2_(p)R4_(q)R5_(r)(wherein R1 is one or more kind elements selected from a groupconsisting of rare earth metals including Y; R2 is one or more kindelements selected from a group consisting of Mg, Ca, Sr, and Ba; R4 isone or more kind elements selected from a group consisting of Ni, Co,Cr, Fe, Cu, Zn, Si, Sn, V, Nb, Ta, Ti, Zr, and Hf R5 is one or two kindelements selected from Mn and Al; and n, p, q, and r satisfy 16≦n≦23;2≦p≦8; 68.5≦q≦76; 1≦r≦6.5; and n+p+q+r=100).

In terms of saving material cost, the hydrogen storage alloy accordingto the invention, misch metals (including La, Ce, Nd, and Pr) arepreferable to be used as raw materials. Use of the misch metals as rawmaterials, the use amounts of costly high purity materials such asneodymium and praseodymium can be suppressed and at the same time sameeffect as that in the case of using the high purity materials can becaused.

In the case the misch metals are used as raw materials, the ceriumcontent in the hydrogen storage alloy of the invention is preferable tobe 2.2 mol % or less. When the cerium content is adjusted to be 2.2 mol% or less, decrease of the cycle life can be suppressed. The effect ofsuppressing the decrease of the cycle life become significant when thecerium content is 1.3 mol % or less. Particularly, when the ceriumcontent is adjusted to be 0.9 mol % or less, the decrease of the cyclelife can be suppressed to an extremely low level.

Further, in the case where the above-mentioned misch metals are used asraw materials, in the hydrogen storage alloy of the invention, it ispreferable that the total ratio of the Pr₅Co₁₉ phase, Ce₅Co₁₉ phase, andCe₂Ni₇ phase is 95% by weight or higher. With such configuration, anexcellent cycle life is exhibited. Particularly, in the case where thetotal ratio of these three phases is 98% by weight or higher, the effectbecomes furthermore significant. Such an effect is supposedly attributedto the suppression of pulverization owing to the uniform alloystructure.

Next, a method for producing a hydrogen storage alloy of the inventionwill be described.

A method for producing the first hydrogen storage alloy involves amelting step of melting alloy raw materials mixed at a prescribedcomposition ratio, a cooling step of rapid solidification the moltenalloy raw materials at a cooling speed of 1000 K/s or higher, and anannealing step of annealing the cooled alloy at temperature range of860° C. or higher and 1000° C. or lower in inert gas atmosphere inpressurized state.

Herein, in the case the composition ratio of the alloy raw materials isdefined by a general formula R1_(s)R2_(b)R3_(c) (wherein R1 is one ormore kind elements selected from a group consisting of rare earth metalsincluding Y; R2 is one or more kind elements selected from a groupconsisting of Mg, Ca, Sr, and Ba; R3 is one or more kind elementsselected from a group consisting of Ni, Co, Mn, Al, Cr, Fe, Cu, Zn, Si,Sn, V, Nb, Ta, Ti, Zr, and Hf), s, b, and c satisfy 8≦s≦30; 1≦b≦10;65≦c≦90; and s+b+c=100.

If the hydrogen storage alloy is produced by such a production method,it is made possible to obtain the hydrogen storage alloy having two ormore layered crystal phases having crystal structures different from oneanother.

To explain it more concretely, at first, based on the chemicalcomposition of the aimed hydrogen storage alloy, prescribed amount of araw material ingot (an alloy raw material) is weighed.

In the melting step, the above-mentioned alloy raw material is put in acrucible and heated at, for example, 1200° C. or higher and 16000° C. orlower, to melt the alloy raw material using a high frequency meltingfurnace in an inert gas atmosphere or vacuum.

In the cooling step, the melted alloy raw material is cooled andsolidified. The cooling speed is preferably 1000 K/s or higher (alsoreferred to as quenching). Quenching at 1000 K/s or higher is effectiveto make the alloy structure very fine and uniform. Further, the coolingspeed can be set in a range of 1,000,000 K/s or lower.

As the cooling method, practically, a melt spinning method at a coolingspeed of 100,000 K/s or higher and a gas atomization method at a coolingspeed of about 10,000 K/s or higher can preferably be employed.

In the annealing step, in the pressurized state in inert gas atmosphere,heating at 860° C. or higher and 1000° C. or lower may be carried outusing, for example, an electric furnace or the like. As the pressurizingcondition, it is preferable to be 0.2 MPa (gauge pressure) or higher and1.0 MPa (gauge pressure) or lower. Further, the treatment time for theannealing step is preferably 3 hours or longer and 50 hours or shorter.

Such an annealing step is effective to release strains of crystallattice and the hydrogen storage alloy subjected to the annealing stepfinally becomes the hydrogen storage alloy comprising two or morelayered crystal phases having crystal structures different from oneanother.

A method for producing the second hydrogen storage alloy is the methodfor producing according to the first hydrogen storage alloy in which thetemperature condition of the annealing step is adjusted to be 890° C. orhigher and 970° C. or lower.

Adjustment of the condition in such a manner makes diffusion of atomsrelatively easy, suppresses evaporation of Mg, Ca, Sr, and Ba, easilyuniformalizes the length of the a-axis in the respective crystal phases,and gives the hydrogen storage alloy having the difference of 0.03 Åbetween the maximum value and the minimum value.

In the method for producing the second hydrogen storage alloy, thetemperature condition in the annealing step is preferable to be 900° C.or higher and 940° C. or lower. Adjustment of the condition in such amanner is effective to make the concentration distribution of respectiveconstituent elements uniform and make the difference of the a-axislength among the produced phases further shorter.

A method for producing the third hydrogen storage alloy is the methodfor producing according to the first hydrogen storage alloy in which theratio of R1 and R2 elements of the alloy raw material and the ratio ofR4 and R5 elements are adjusted and the above-mentioned average atomradium rA (Å) and the ratio (B/A) are satisfied in the case ofconsideration of the atom radius of the respective elements.

Further, in the method for producing the third hydrogen storage alloy,the average atom radium rA a) and the ratio (B/A) are preferable tosatisfy 1.810≦rA≦1.825 and 3.60≦(B/A)≦3.70.

Adjustment of the condition in such a manner is effective to maintainthe high capacity and at the same time to give further improved cycleperformance.

A method for producing the fourth hydrogen storage alloy is the methodfor producing the first hydrogen storage alloy in which the alloy rawmaterial composition is adjusted to give the composition of the fourthhydrogen storage alloy and the temperature condition in the annealingstep is adjusted to be 890° C. or higher and 970° C. or lower.

Adjustment of the condition in such a manner makes diffusion of atomrelatively easy and effective to suppress evaporation of Mg, Ca, Sr, andBa, satisfy the prescribed composition, and obtain the hydrogen storagealloy containing the crystal phase having the Ce₅Co₁₉ type crystalstructure.

Further, in the method for producing the fourth hydrogen storage alloy,it is more preferable that in the above-mentioned compositionLa_(h)R6_(i)R7_(j)Mg_(k)R8_(m), k satisfies 3.4<k<4.3; R7 consists ofone or more kind elements R7′ selected from a group consisting of Zr,Zn, and Sn in combination with Ti and is defined as R7=Ti_(t)R7′_(j-t)(wherein 0≦t<0.3); and R8 consists of one or more kind elements R8′selected from a group consisting of Ni, Co, Cu, Fe, and Cr incombination with Mn and defined as R8=Mn_(s)R8′_(m-s) (wherein 0<s<1.1).

Adjustment of the condition in such a manner is effective to maintainthe high capacity and at the same time to further improve the cycleperformance.

The method for producing the fifth hydrogen storage alloy is the methodfor producing according to the first hydrogen storage alloy in which thetemperature condition in the annealing step is adjusted to be 890° C. orhigher and 970° C. or lower.

Adjustment of the condition in such a manner makes diffusion of atomrelatively easy and effective to suppress evaporation of Mg, Ca, Sr, andBa, satisfy the prescribed composition, and obtain the hydrogen storagealloy containing 20% by weight or less of the crystal phase having theCaCu₅ type crystal structure.

In the method for producing the fifth hydrogen storage alloy, it is morepreferable that in the composition La_(u)R9_(v)Mg_(w)R10_(x)R11_(y)(wherein, R9 is at least one element of Pr and Nd; R10 is at least oneelement of Ni and Co; R11 is at least one element of Al and Mn; and u,v, w, x, and y satisfy 4.255≦u≦17.39; 0≦v≦13.62; 2.128≦w≦4.701;72.30≦x≦77.66; and 1.06≦y≦6.38).

Adjustment of the condition in such a manner is effective to make Mg,Mn, and Al occupy specified atom sites in the layered structure,stabilize their structure, and suppress production of the crystal phasehaving the CaCu₅ type crystal structure.

A hydrogen storage alloy electrode of the invention is provided with theabove-mentioned hydrogen storage alloy as a hydrogen storage medium. Atthe time of using the hydrogen storage alloy of the invention for anelectrode as a heat storage medium, it is preferable to pulverize thehydrogen storage alloy for the use.

The pulverization of the hydrogen storage alloy at the time of electrodeproduction may be carried out either before or after annealing, however,since the surface area becomes wide by the pulverization, in terms ofprevention of surface oxidation of the alloy, it is desirable topulverize the alloy after annealing. The pulverization is preferable tocarry out in inert atmosphere for oxidation prevention of the alloysurface.

The pulverization may be carried out by, for example, mechanicalpulverization, hydrogenation pulverization, and the like.

Further, a secondary battery of the invention is configured to be anickel-metal hydride battery using the hydrogen storage alloy as anegative electrode. Since the hydrogen storage alloy of the invention,that is the hydrogen storage alloy electrode, has corrosion resistanceto an aqueous strongly alkaline solution to be used as an electrolyticsolution of a nickel-metal hydride battery or the like, it is excellentin the cycle performance in the case where hydrogen absorption andrelease are repeatedly carried out. As a result, the charge anddischarge cycle performance of the secondary battery also becomeexcellent.

In addition, as a positive electrode of the secondary battery, forexample, nickel electrode (sintered type or non-sintered type) isemployed.

EXAMPLES

Hereinafter, the invention will be described more practically, referringto Examples and Comparative Examples; however the invention should notbe limited to the following Examples.

Example 1

A prescribed amount of a raw material ingot having the chemicalcomposition shown in Table 1 was weighed, put in a crucible, and heatedat 1500° C. in reduced pressure argon atmosphere using a high frequencymelting furnace to melt the material. After the melting, the meltedalloy was quenched by employing a melt spinning method and solidified.

Next, the obtained alloy was heated at 910° C. in 0.2 MPa (gaugepressure, hereinafter the same) of pressurized argon gas.

Comparative Example 1

A prescribed amount of a raw material ingot having the chemicalcomposition shown in Table 1 was weighed, put in a crucible, and heatedat 1500° C. in reduced pressure argon atmosphere using a high frequencymelting furnace to melt the material. After the melting, the meltedalloy was quenched by employing a melt spinning method and solidified.

Next, the obtained alloy was heated at 910° C. in 0.2 MPa (gaugepressure, hereinafter the same) of pressurized argon gas.

TABLE 1 La Pr Mg Ni Co Al Example 1 13.3 4.2 3.3 71.9 4.2 3.1Comparative 12.8 4.3 4.3 68.1 6.4 4.3 Example 1

<Measurement of Crystal Structure and Calculation of Existence Ratio>

Each obtained hydrogen storage alloy was pulverized to obtain a powderwith an average particle diameter (D50) of 20 μm and the powder wassubjected to measurement under condition of 40 kV and 100 mA (Cu bulb)using an X-ray diffraction apparatus (manufactured by Bruker AXS: modelnumber M06XCE). Further, as structure analysis, analysis by Rietveldmethod (analysis software: RIETAN 2000) was carried and the productionratios of the produced crystal phases in each hydrogen storage alloywere calculated.

The ratios (% by weight) of the produced phases are shown in Table 2.

TABLE 2 Ce₅Co₁₉ Pr₅Co₁₉ CaCu₅ AuBe₅ phase phase phase phase TotalExample 1 62.7 28.5 8.74 0.0 100 Comparative 69.1 16.1 11.8 3.0 100example 1

<Evaluation of Distribution State of Crystal Phase>

With respect to the obtained hydrogen storage alloy powders of Exampleand Comparative Example, the distribution state (color map) of Ni and Mgwas observed using EPMA (Electron Probe Micro Analyzer) to evaluate thedistribution state of produced phases. With respect to Example 1 andComparative Example 1, FIG. 4 shows photographs of the distributionstate (color map) of Ni and Mg obtained by EPMA.

As shown in FIG. 4, it is observed that both Ni and Mg were distributeduniformly in the hydrogen storage alloy of Example and according toRietveld analysis result or the like for the alloy, it was confirmedthat a plurality of phases were produced and as comprehensive analysisof these results, it could be confirmed that the crystal phases existedin layered state in the hydrogen storage alloy. On the other hand, inthe hydrogen storage alloy of Comparative Example, Ni and Mg weredistributed ununiformly in places and thus it was supposed that thecrystal phases separately existed without forming a layered body. Inaddition, when the lattice image of the hydrogen storage alloy ofExample 1 was observed by transmission electron microscope (TEM), thecrystal phases with the crystal structures different from one anotherwere layered in the c-axis direction.

<Evaluation of Cycle Performance> (a) Production of Electrode

After 3 parts by weight of a nickel powder (manufactured by INCO, #210)was added to 100 parts by weight of each of the obtained hydrogenstorage alloy powders of Example and Comparative Example, an aqueoussolution in which a thickener (methyl cellulose) was dissolved was addedand further 1.5 parts by weight of a binder (styrene-butadiene rubber)was added to obtain a paste which was applied to both faces of a punchedsteel plate (porosity 60%) with a thickness of 45 μm and dried andthereafter pressed in 0.36 mm thickness to obtain a negative electrode.On the other hand, a sintered type nickel hydroxide electrode with anexcess capacity was used as a positive electrode.

(b) Production of Flooded Cell

The negative electrode produced in the above-mentioned manner wassandwiched with positive electrodes while inserting separators betweenthem and these electrodes were fixed with bolts at pressure of 1 kgf/cm²to assemble an opened type cell. A mixed solution of a 6.8 mol/L KOHsolution and a 0.8 mol/L LiOH solution was used as an electrolytesolution.

(c) Measurement Method of Discharge Capacity

In a water bath at 20° C., charging and discharging was repeated 65cycles in condition of charging at 0.1 ItA to 150%, discharging at 0.2ItA to cut off voltage of −0.6 V (vs. Hg/HgO). The results are shown inFIG. 5.

As shown in FIG. 5, with respect to the hydrogen storage alloy ofComparative Example 1 in which the crystal phases existed separately,the discharge capacity was lowered to about 88% after the 65 cycles,whereas with respect to the hydrogen storage alloy of Example 1, it wasconfirmed that the discharge capacity was maintained at 99.7% even after65 cycles.

Examples 2 to 6

Using the alloy materials with the compositions shown in the followingTable 3, hydrogen storage alloys of Examples 2 to 6 were produced in thesame manner as Example 1. When the lattice images of these hydrogenstorage alloys were observed by a transmission electron microscope(TEM), it was confirmed that crystal phases with different crystalstructures were layered in the c-axis direction.

TABLE 3 Unit: mol % La Pr Mg Ni Co Mn Al Example 2 12.58 4.19 4.19 71.282.10 2.10 2.10 Example 3 17.19 0.00 3.77 68.13 6.29 0.00 3.14 Example 49.64 8.18 3.14 75.89 0.00 0.00 1.68 Example 5 4.30 14.46 2.09 77.50 1.650.00 0.00 Example 6 5.01 14.08 2.12 74.50 4.29 0.00 0.00

<Measurement of A-Axis Length>

With respect to each of the obtained hydrogen storage alloys, theproduction ratios of the crystal phases were calculated and at the sametime, the XRD patterns were measured by an X-ray diffraction apparatusand the a-axis length for each produced crystal phase was calculated byRietveld method (analysis software: RIETAN 2000). The results are shownin the following Table 4 and FIG. 6.

Further, in the same manner as Example 1, the ratios of the producedphases of each hydrogen storage alloy and the retention ratio of thedischarge capacity after 50 cycles were measured. The results are alsoshown in Table 4.

TABLE 4 Ratios of produced crystal phase A-axis Capacity (% by weight)[A-axis length (Å)] length retention Ce₂Ni₇ Ce₅Co₁₉ Pr₅Co₁₉ CaCu₅difference ratio phase phase phase phase (Å) (%) Exam- 36.95 38.26 14.0410.75 0.003 93.42 ple 2 [5.045] [5.044] [5.044] [5.043] Exam- 26.4335.11 20.42 18.04 0.016 92.87 ple 3 [5.066] [5.088] [5.059] [5.050]Exam- 42.32 37.89 13.13 6.67 0.036 90.25 ple 4 [5.028] [5.017] [5.020][4.992] Exam- 32.9 40.0 12.6 14.5 0.033 90.3 ple 5 [5.025] [5.015][5.022] [4.992] Exam- 51.3 30.6 8.50 9.60 0.030 93.4 ple 6 [5.033][5.029] [5.034] [5.004]

Examples 7 to 42

Using the alloy raw materials of the compositions shown in the followingTable 5, hydrogen storage alloys of Examples 7 to 42 were produced inthe same manner as Example 1. When the lattice images of these hydrogenstorage alloys were observed by a transmission electron microscope(TEM), it was confirmed that crystal phases with different crystalstructures were layered in the c-axis direction.

Comparative Example 2

Similarly, using the allow raw material shown in Table 5, a hydrogenstorage alloy of Comparative Example 2 was produced in the same manneras Comparative Example 1.

TABLE 5 Main Raw material composition produced rA La Ce Pr Nd Sm Y Mg NiCo Mn Al B/A phase Å Example 7 12.4 0.0 4.1 0.0 0.0 0.0 4.1 71.1 4.1 1.03.1 3.85 Ce2Ni7 1.8122 Example 8 12.8 0.0 4.3 0.0 0.0 0.0 4.3 70.2 4.31.1 3.2 3.70 Pr5Co19 1.8122 Example 9 12.9 0.0 4.3 0.0 0.0 0.0 4.3 69.94.3 1.1 3.2 3.65 Pr5Co19 1.8122 Example 10 13.2 0.0 4.4 0.0 0.0 0.0 4.469.1 4.4 1.1 3.3 3.53 Pr5Co19 1.8122 Example 11 13.4 0.0 4.5 0.0 0.0 0.04.5 68.8 4.5 1.1 3.3 3.48 Ce2Ni7 1.8122 Example 12 13.8 0.0 3.1 0.0 0.00.0 3.7 71.1 4.1 1.0 3.1 3.85 Ce5Co19 1.8202 Example 13 16.5 0.0 2.1 0.00.0 0.0 2.1 71.1 4.1 1.0 3.1 3.85 Pr5Co19 1.8446 Example 14 17.2 0.0 0.00.0 0.0 0.0 4.3 71.0 4.3 0.0 3.2 3.65 Ce5Co19 1.822 Example 15 14.1 0.03.7 0.0 0.0 0.0 3.9 71.7 4.3 0.0 2.2 3.60 Pr5Co19 1.8192 Example 16 13.00.0 0.0 4.3 0.0 0.0 4.3 72.8 2.2 0.0 3.3 3.60 Pr5Co19 1.8108 Example 1713.0 0.0 0.0 5.2 0.0 0.0 3.5 72.8 2.2 0.0 3.3 3.60 Pr5Co19 1.8196Example 18 9.8 0.0 6.5 1.1 0.0 0.0 4.3 71.7 4.3 0.0 2.2 3.60 Pr5Co191.8045 Example 19 6.5 0.0 10.9 0.0 0.0 0.0 4.3 71.7 4.3 0.0 2.2 3.60Ce2Ni7 1.7975 Example 20 12.8 0.0 0.0 3.2 0.0 0.0 5.3 72.3 4.3 0.0 2.13.70 PuNi3 1.7999 Example 21 12.8 0.0 0.0 5.3 0.0 0.0 3.2 72.3 4.3 0.02.1 3.70 Pr5Co19 1.8218 Example 22 17.9 0.0 0.0 1.1 0.0 0.0 2.3 72.3 4.30.0 2.1 3.70 Pr5Co19 1.844 Example 23 3.9 8.7 1.1 4.3 0.2 0.0 3.5 71.74.3 0.0 2.2 3.60 Ce2Ni7 1.7978 Example 24 14.0 0.0 0.0 2.1 0.0 2.1 3.073.4 2.1 1.1 2.1 3.70 Ce5Co19 1.8253 Example 25 12.5 0.0 0.0 0.0 0.0 4.24.2 69.8 6.3 0.0 3.1 3.80 Ce5Co19 1.8068 Example 26 12.9 0.0 0.0 0.0 0.04.6 3.3 69.8 6.3 0.0 3.1 3.80 Ce5Co19 1.8163 Example 27 14.8 0.2 0.0 0.00.0 2.9 2.9 69.8 6.3 0.0 3.1 3.80 Ce5Co19 1.8273 Example 28 15.8 0.0 0.00.0 0.0 2.5 2.5 69.8 6.3 0.0 3.1 3.80 Ce2Ni7 1.8349 Example 29 14.8 2.10.0 0.0 0.0 0.0 3.7 73.2 4.1 0.0 2.1 3.85 Ce5Co19 1.8223 Example 30 14.52.0 0.0 0.0 0.0 0.0 3.6 73.7 4.0 0.0 2.0 3.95 Ce5Co19 1.8223 Example 3116.1 0.0 2.2 0.0 0.0 0.0 4.0 69.9 4.5 0.0 3.3 3.48 Ce2Ni7 1.8226 Example32 11.0 0.0 0.0 6.6 0.0 0.0 4.4 69.1 5.5 0.0 3.3 3.53 Ce5Co19 1.8052Example 33 11.2 0.0 0.0 6.7 0.0 0.0 4.5 69.9 4.5 0.0 3.3 3.48 Ce5Co191.8052 Example 34 12.8 0.0 4.3 0.0 0.0 0.0 4.3 70.2 4.3 0.0 4.3 3.70Ce5Co19 1.8122 Example 35 16.3 0.0 0.0 0.0 0.0 0.0 5.4 71.7 4.3 0.0 2.23.60 PuNi3 1.8083 Example 36 13.9 0.0 0.0 4.8 0.0 0.0 3.0 71.4 4.3 0.02.6 3.62 Pr5Co19 1.8262 Example 37 13.9 0.0 0.0 4.3 0.0 0.0 3.5 71.4 4.30.0 2.6 3.62 Pr5Co19 1.8218 Example 38 14.1 0.0 0.0 4.3 0.0 0.0 3.2 71.44.3 0.0 2.6 3.62 Pr5Co19 1.8246 Example 39 15.2 0.0 0.0 3.2 0.0 0.0 3.271.4 4.3 0.0 2.6 3.62 Pr5Co19 1.8274 Example 40 15.4 0.0 0.0 3.3 0.0 0.03.3 71.1 4.4 0.0 2.6 3.56 Pr5Co19 1.8274 Example 41 16.4 0.0 0.0 2.2 0.00.0 3.3 71.1 4.4 0.0 2.6 3.56 Pr5Co19 1.8302 Example 42 3.8 0.0 7.6 7.60.0 0.0 3.3 73.3 0.0 0.0 4.4 3.50 Ce2Ni7 1.8001 Comparative 11.5 3.9 0.20.8 0.0 0.0 0.0 67.2 4.9 6.6 4.9 5.10 CaCu5 1.8612 example 2 CapacityCapac- retention E- Produced phase (%) ity ratio valu- Ce2Ni7 Pr5Co19Ce5Co19 La4MgNi24 CaCu5 Others mAh/g % ation Example 7 43 6 32 5 14 35189.3 Example 8 18 34 21 21 6 352 94.5  Example 9 28 50 12 6 4 349 96 Example 10 40 42 10 4 4 347 95.4  Example 11 50 30 9 11 340 91.6 ◯Example 12 32 17 44 7 354 92.9 ◯ Example 13 55 33 12 325 88.6 Δ Example14 20 28 42 10 355 91.3 ◯ Example 15 20 42 34 4 350 97  Example 16 1045 40 5 348 97.5  Example 17 26 60 14 347 97.8  Example 18 36 40 20 4350 96.5  Example 19 45 18 30 7 324 94.6 ◯ Example 20 15 19 32 34 32190.6 Δ Example 21 58 36 6 354 97.5  Example 22 24 55 17 4 337 92.6 ΔExample 23 39 19 32 10 325 88 Δ Example 24 30 28 31 6 5 347 96  Example25 22 15 40 10 13 360 92.7 ◯ Example 26 20 19 34 7 20 355 94.9  Example27 18 22 31 5 6 18 350 94.1  Example 28 35 25 30 10 331 90.4 Δ Example29 16 20 48 16 358 91.5 ◯ Example 30 20 58 22 352 91.3 ◯ Example 31 5514 21 10 326 88.6 Δ Example 32 31 23 39 7 341 94.1  Example 33 37 11 4111 331 92.1 Δ Example 34 18 38 37 7 337 93 ◯ Example 35 21 13 29 37 34188.9 ◯ Example 36 55 38 7 346 94.9  Example 37 7 47 43 3 348 95.2 Example 38 50 45 5 347 95  Example 39 53 44 3 352 95.3  Example 40 847 42 3 351 95.3  Example 41 39 26 35 357 90.6 ◯ Example 42 92 3 5 33792.6 Δ Comparative 100 307 97.5 ◯ example 2

With respect to the hydrogen storage alloys of Examples 7 to 42 andComparative Example 2, the production ratios of the crystal phases werecalculated and cycle performance measurement was carried out as same asExample 1. The maximum values of the discharge capacity and retentionratios of discharge capacity at 50 cycles are shown in Table 5. Thosehaving the maximum values of the discharge capacity of 340 mAh/g orhigher and retention ratio of the discharge capacity at 93% or higherare marked with : those which satisfy either one were marked with ∘:and those which satisfy neither were marked with Δ. The results areshown together in Table 5.

Further, the calculation results of average atom radius rA (Å) of theabove-mentioned R1. element and R2 element (elements at A side) and theratio (B/A) are shown together in Table 5 and the graph formed byplotting the evaluation results in the B/A-rA (Å) coordinate is shown inFIG. 7.

As shown in FIG. 7, in the case of using the hydrogen storage alloys ofExamples which satisfy 3.53≦(B/A)≦3.80 and0.0593(B/A)+1.59≦rA≦0.0063(B/A)+1.81, that almost all of them exhibitedexcellent discharge capacity and cycle performance.

Further, from Table 5, in the case rA and B/A satisfy theabove-mentioned expressions, respectively, the main produced phase tendsto be Pr₅Co₁₉ phase or Ce₅Co₁₉ phase and it was confirmed that excellentdischarge capacity and cycle performance were exhibited.

Furthermore, from Table 5, with respect to Examples 15 to 18, 21, 24 to27, 32, 34, and 35 to 40 which satisfy the following: the R1 is one ormore kind elements R1′ selected from a group consisting of Ce, Pr, Nd,Sm, and Y and La at La/R1′ ratio of 5 or less; the R2 is Mg; the R4 isone or two elements selected from Ni and Co; the R5 is Al; and d, e, f,and g respectively satisfy 16≦d≦19; 2≦e≦5; 73≦f≦78; and 2≦g≦4, Pr5Co₁₉phase and Ce₅Co₁₉ phase were produced preferentially and the uniformityof the alloys was improved.

Examples 43 to 81

Using the alloy raw materials of the compositions shown in the followingTable 6, hydrogen storage alloys of Examples 43 to 81 were produced inthe same manner as Example 1. When the lattice images of these hydrogenstorage alloys were observed by a transmission electron microscope(TEM), it was confirmed that crystal phases with different crystalstructures were layered in the c-axis direction.

With respect to the obtained hydrogen storage alloys, the productionratios of the crystal phases were calculated and cycle performance (themaximum values of the discharge capacity and retention ratios ofdischarge capacity at 50 cycles) measurement was carried out as same asExample 1. The results are shown together in Table 6. Theabove-mentioned La/R1′ ratios are also shown in Table 6.

TABLE 6 Alloy composition (mol %) La Ce Pr Nd Y Zr Ti V Sn Zn Mg Ni CoMn Al Fe Cu Si La/R1′ Example 8.3 8.5 0.2 4.3 68.1 6.4 2.1 2.1 100 0.4943 Example 11.7 5.1 0.2 4.3 68.1 6.4 2.1 2.1 100 0.70 44 Example 13.63.2 0.2 4.3 68.1 6.4 2.1 2.1 100 0.81 45 Example 14.3 2.6 0.2 4.3 68.16.4 2.1 2.1 100 0.85 46 Example 14.7 2.1 0.2 4.3 68.1 6.4 2.1 2.1 1000.87 47 Example 16.8 0.0 0.2 4.3 68.1 6.4 2.1 2.1 100 1.00 48 Example17.9 2.1 0.2 1.1 68.1 6.4 2.1 2.1 100 0.89 49 Example 16.8 2.1 0.2 2.168.1 6.4 2.1 2.1 100 0.89 50 Example 16.2 2.1 0.2 2.8 68.1 6.4 2.1 2.1100 0.88 51 Example 13.6 2.1 0.2 5.3 68.1 6.4 2.1 2.1 100 0.86 52Example 12.6 2.1 0.2 6.4 68.1 6.4 2.1 2.1 100 0.86 53 Example 13.4 4.30.2 3.4 68.1 6.4 2.1 2.1 100 0.76 54 Example 14.5 3.2 0.2 3.4 68.1 6.42.1 2.1 100 0.82 55 Example 14.5 3.2 0.2 3.4 68.1 6.4 2.1 2.1 100 0.8256 Example 14.5 3.2 0.2 3.4 68.1 6.4 2.1 2.1 100 0.82 57 Example 14.53.2 0.2 3.4 68.1 6.4 2.1 2.1 100 0.82 58 Example 14.5 3.4 3.4 68.1 6.42.1 2.1 100 0.81 59 Example 14.5 2.8 0.6 3.4 68.1 6.4 2.1 2.1 100 0.8460 Example 14.5 2.3 1.1 3.4 68.1 6.4 2.1 2.1 100 0.86 61 Example 14.53.2 0.2 3.4 68.1 6.4 2.1 2.1 100 0.82 62 Example 14.5 3.2 0.2 3.4 68.16.4 2.1 2.1 100 0.82 63 Example 14.5 3.2 0.2 3.4 68.1 6.4 2.1 2.1 1000.82 64 Example 16.6 3.7 0.2 3.9 63.4 7.3 2.4 2.4 100 0.82 65 Example15.1 3.3 0.2 3.6 66.7 6.7 2.2 2.2 100 0.82 66 Example 14.2 3.1 0.2 3.368.8 6.3 2.1 2.1 100 0.82 67 Example 13.3 2.9 0.2 3.1 70.6 5.9 2.0 2.0100 0.82 68 Example 14.5 3.2 0.2 3.4 68.1 6.4 4.3 100 0.82 69 Example14.5 3.2 0.2 3.4 68.1 4.3 2.1 2.1 2.1 100 0.82 70 Example 14.5 3.2 0.23.4 68.1 4.3 2.1 2.1 2.1 100 0.82 71 Example 14.5 3.2 0.2 3.4 68.1 4.32.1 2.1 2.1 100 0.82 72 Example 14.8 3.3 0.2 3.5 69.6 4.3 1.1 3.3 1000.82 73 Example 14.5 3.2 0.2 3.4 70.2 4.3 1.1 3.2 100 0.82 74 Example14.2 3.1 0.2 3.3 70.8 4.2 1.0 3.1 100 0.82 75 Example 13.9 3.1 0.2 3.371.4 4.1 1.0 3.1 100 0.82 76 Example 13.6 3.0 0.2 3.2 72.0 4.0 1.0 3.0100 0.82 77 Example 14.9 4.0 0.0 3.1 74.7 0.0 3.3 100 0.79 78 Example14.9 3.5 0.0 3.5 70.3 4.4 3.3 100 0.81 79 Example 14.6 3.4 0.0 3.4 68.84.3 2.2 3.2 100 0.81 80 Example 4.7 8.1 7.0 0.0 3.5 72.1 2.3 0.0 2.3 1000.24 81 Capacity Produced phases (% by weight) Capacity retention AB2Ce2Ni7 Gd2Co7 Pr5Co19 Ce5Co19 CaCu5 Others Total mAh/g ratio % Example 341 8 33 15 100 336 94.9 43 Example 1 39 10 40 10 100 360 96.4 44 Example41 9 41 9 100 364 96.3 45 Example 38 12 37 13 100 368 96.1 46 Example 3014 37 19 100 370 94 47 Example 26 15 35 24 100 365 93.1 48 Example 46 1430 10 100 125 99.7 49 Example 45 17 29 9 100 252 99.5 50 Example 44 1431 11 100 335 98.1 51 Example 20 35 45 100 331 93.6 52 Example 2 42 56100 285 93.5 53 Example 30 66 4 100 363 97 54 Example 31 64 5 100 36897.1 55 Example 23 21 48 8 100 357 95.1 56 Example 24 70 6 100 368 97 57Example 4 4 20 62 10 100 369 95.9 58 Example 31 66 3 100 372 95 59Example 5 24 52 17 2 100 351 96.9 60 Example 10 21 42 23 4 100 336 96.161 Example 31 64 5 100 361 96.9 62 Example 31 64 5 100 358 96.8 63Example 31 64 5 100 361 97 64 Example 43 5 31 21 100 335 95.9 65 Example61 18 21 100 342 97.1 66 Example 24 20 45 11 100 372 95.7 67 Example 338 29 30 100 375 95.1 68 Example 33 24 31 6 6 100 356 97.3 69 Example 3244 21 3 100 346 95.1 70 Example 30 40 26 4 100 344 95.2 71 Example 28 4620 6 100 340 96.1 72 Example 63 6 24 7 100 364 97.3 73 Example 55 4 3110 100 366 97.1 74 Example 44 17 30 9 100 366 96.4 75 Example 24 21 4213 100 361 95.4 76 Example 20 28 30 22 100 350 95 77 Example 28 55 12 5100 366 97.4 78 Example 28 55 12 5 100 366 97.4 79 Example 33 44 12 7 4100 364 97.1 80 Example 100 100 330 97 81

As shown in Table 6, when the hydrogen storage alloys containing thecrystal phase having the Ce₅Co₁₉ type crystal structure and having acomposition defined as La_(h)R6_(i)R7_(i)Mg_(k)R8m (wherein R6 is one ormore kind elements selected from a group consisting of rare earth metalsincluding Y and excluding La; R7 is one or more kind elements selectedfrom a group consisting of Zr, Ti, Zn, Sn and V; R8 is one or more kindelements selected from a group consisting of Ni, Co, Mn, Al, Cu, Fe, Cr,and Si; and h, i, j, k and m satisfy 0≦j≦0.65; 2≦k≦5.5;0.70≦h/(h+i)≦0.85; and h+i+j+k+m=100), that is, the hydrogen storagealloy of Examples 44 to 46, Examples 54 to 60, Examples 62 to 64, andExamples 66 to 80, were used, it was confirmed that excellent dischargecapacity and cycle performance were exhibited.

Examples 82 to 91

Using the alloy raw materials of the compositions shown in the followingTable 7, hydrogen storage alloys of Examples 82 to 91 were produced inthe same manner as Example 1. When the lattice images of these hydrogenstorage alloys were observed by a transmission electron microscope(TEM), it was confirmed that crystal phases with different crystalstructures were layered in the c-axis direction.

Further, with respect to the hydrogen storage alloys, the productionratios of the crystal phases were calculated and cycle performancemeasurement was carried out as same as Example 1. The results are showntogether in Table 7.

TABLE 7 Capac- ity reten- tion Composition ratio Produced phases La PrNd Y Mg Ni Co Mn Al (%) CaCu5 Ce2Ni7 Gd2Co7 Ce5Co19 Pr5Co19 La5MgNi24AuBe5 Example 82 17.0 0.0 0.0 0.0 4.3 68.1 6.4 0.0 4.3 87.5 36.9 0.0 0.037.7 14.9 0.0 10.5 Example 83 17.9 0.0 0.0 0.0 3.4 68.1 6.4 2.1 2.1 88.625.9 40.3 0.0 24.9 5.9 0.0 3.0 Example 84 17.0 0.0 0.0 0.0 4.3 67.0 6.42.1 3.2 88.2 35.2 12.8 0.0 36.2 9.7 0.0 6.0 Example 85 14.9 0.0 0.0 2.14.3 68.1 6.4 2.1 2.1 97.9 1.2 8.7 0.0 23.7 53.1 13.4 0.0 Example 86 12.80.0 5.1 0.0 3.4 67.0 8.5 0.0 3.2 96.0 2.9 13.1 0.0 17.0 67.0 0.0 0.0Example 87 12.6 4.2 0.0 0.0 4.2 69.5 6.3 0.6 2.5 96.0 9.8 0.0 0.0 22.26.9 61.1 0.0 Example 88 12.8 4.3 0.0 0.0 4.3 68.1 6.4 0.0 4.3 94.2 11.80.0 0.0 69.1 9.0 7.1 3.0 Example 89 12.8 4.3 0.0 0.0 4.3 61.7 12.8 0.04.3 92.5 14.1 0.0 0.0 60.1 10.1 10.0 5.7 Example 90 12.4 4.1 0.0 0.0 4.169.1 6.2 2.1 2.1 91.6 21.7 42.9 0.0 19.7 15.7 0.0 0.0 Example 91 9.4 7.30.0 0.0 4.2 68.8 6.3 2.1 2.1 91.7 22.0 39.8 13.4 24.9 0.0 0.0 0.0

Further, with respect to the hydrogen storage alloys, based on theresults of Table 7, a graph showing the relation of the capacityretention ratio to the production ratio of CaCu₅ phase is shown in FIG.8.

As shown in FIG. 8, in the case of using the hydrogen storage alloyswith 22% by weight or less of CaCu₅ phase, that is, the hydrogen storagealloys of Examples 85 to 91, it was confirmed that the capacityretention ratios were further higher values and particularly, in thecase the CaCu₅ phase was 5% by weight or less, it was confirmed that thecapacity retention ratios became extremely high values.

Examples 92 to 101

Using the alloy raw materials of the compositions shown in the followingTable 8, hydrogen storage alloys of Examples 92 to 101 were produced inthe same manner as Example 1. When the lattice images of these hydrogenstorage alloys were observed by a transmission electron microscope(TEM), it was confirmed that crystal phases with different crystalstructures were layered in the c-axis direction.

Further, with respect to the hydrogen storage alloys, the productionratios of the crystal phases were calculated in the same manner asExample 1. Furthermore, with respect to the hydrogen storage alloys,using Siebert PCT measurement apparatus (manufactured by Suzuki SyokanCo. Ltd., P73-07), the equilibrium pressure at 80° C. in case of H/M=0.5of PCT curve (pressure-composition isothermal curve) was calculated.Further, after cells using the respective hydrogen storage alloys wereleft at 45° C. for 14 days, the remaining discharge capacity wasmeasured in the same manner as described above and the remainingdischarge capacity to the maximum discharge capacity was calculated. Theresults are also shown in Table 8.

TABLE 8 Equilibrium Remaining pressure capacity Composition (Mpa) (%) LaPr Nd Y Mg Ni Co Mn Al B/A Example 92 0.035 17.0 0.0 0.0 0.0 4.3 68.16.4 1.7 2.6 3.7 Example 93 0.045 73.51 17.0 0.0 0.0 0.0 4.3 70.2 6.4 1.11.1 3.7 Example 94 0.052 75.17 17.0 0.0 0.0 0.0 4.3 72.3 4.3 2.1 0.0 3.7Example 95 0.064 74.32 12.8 4.3 0.0 0.0 4.3 71.3 4.3 3.2 0.0 3.7 Example96 0.057 76.21 16.7 0.0 0.0 0.0 4.2 68.8 6.3 1.7 2.5 3.8 Example 970.047 74.98 12.8 4.3 0.0 0.0 4.3 68.1 6.4 1.7 2.6 3.7 Example 98 0.1170.72 5.0 13.8 0.0 0.0 2.1 77.1 0.0 0.4 1.7 3.8 Example 99 0.13 69.1112.9 0.0 0.0 4.3 4.3 77.4 0.0 0.0 1.1 3.65 Example 100 0.18 68.45 4.30.0 13.2 0.0 3.8 68.1 6.4 1.1 3.2 3.7 Example 101 0.068 71.33 8.4 8.40.0 0.0 4.2 69.5 4.2 5.3 0.0 3.75 Produced phases Ce2Ni7 Gd2Co7 Ce5Co19Pr5Co19 CaCu5 AuBe5 La5MgNi24 Total Example 92 14.59 0 14.47 9.84 8.03 053.06 100.0 Example 93 39.03 0 40.22 8.37 12.38 0 0 100.0 Example 9436.3 0 32.6 18.4 12.7 0 0 100.0 Example 95 44.47 4.04 28.92 16.29 6.27 00 100.0 Example 96 14.39 0 18.53 8.62 16.34 0 42.12 100.0 Example 9717.88 0 24.38 48.42 6 0 3.33 100.0 Example 98 30.22 0 39.67 13.89 16.230 0 100.0 Example 99 13.05 33.51 43.44 0. 10.02 0 0 100.0 Example 10036.1 0 33.45 8.81 18.9 2.77 0 100.0 Example 101 20.54 0 30.23 29.64 19.60 0 100.0

Further, with respect to the respective hydrogen storage alloys, basedon the results in Table 8, a graph showing the relation of the remainingdischarge capacity to the hydrogen equilibrium pressure is shown in FIG.9.

As shown in FIG. 9, in the case of using the hydrogen storage alloyswith 22% by weight or less of CaCu₅ phase and having hydrogenequilibrium pressure of 0.07 MPa or lower, that is, the hydrogen storagealloys of Examples 92 to 97, it was confirmed that the remainingdischarge capacity was high value.

Examples 102 to 109

Using the alloy raw materials of the compositions shown in the followingTable 9, hydrogen storage alloys of Examples 102 to 109 were produced inthe same manner as Example 1. Herein, in Example 102 and Example 108,respectively high purity materials were used for sources of La, Ce, andNd and in Examples 103 to 107 and Example 109 excluding the former,misch metal including La, Ce, Pr, and Nd was used. When the latticeimages of these hydrogen storage alloys were observed by a transmissionelectron microscope (TEM), it was confirmed that crystal phases withdifferent crystal structures were layered in the c-axis direction.

Next, using these hydrogen storage alloys for negative electrodes,sealed cells were respectively produced and the cycle life was measuredfor each sealed cell. The practical procedure was as described below.

<Production of Negative Electrode>

An aqueous solution in which a thickener (methyl cellulose) wasdissolved and each hydrogen storage alloy powder were mixed and furthermixed with 0.8% by weight of a binder (styrene-butadiene rubber) toobtain a paste which was applied to both faces of a punched steel plate(thickness 35 pm) and dried and thereafter the resulting steel plate waspressed to a prescribed thickness (0.3 mm) to obtain a negativeelectrode.

<Production of Positive Electrode>

An aqueous solution in which a thickener (carboxymethyl cellulose) wasdissolved and paste of an active material were packed in a foamed nickelsubstrate and dried and thereafter, the resulting substrate was pressedto a prescribed thickness (0.78 mm) to obtain a positive electrode. Amaterial used as the active material was a material obtained by coatingthe surface of nickel hydroxide containing 3% by weight of zinc and 0.5%by weight of cobalt in form of a solid solution with 6% by weight ofcobalt hydroxide.

<Production of Sealed Cell>

A jelly roll was produced by spirally rolling the obtained positiveelectrode and negative electrode at a ratio of positive electrodecapacity 1 to negative electrode capacity 1.25 while inserting aseparator between them and a positive electrode terminal part and acurrent collection terminal were resistance welded and thereafter, thejelly roll was housed in a cylindrical metal case. Further, 1.81 ml ofan electrolyte solution containing 8 mol/L KOH and 0.8 mol/L LiOH wasinjected and a cover made of a metal and equipped with a safety valvewas used for closing to produce each sealed cell with 2500 mAh AA size.

<Cycle Test>

After the above-mentioned sealed cell was initially charged at 200C and0.02 It (A) (50 mA) for 10 hours, the cell was again charged at 0.25 It(A) (625 mA) for 5 hours. Thereafter, discharging at 20° C. and 0.2 It(A) (500 mA) to cut off voltage of 1 V and charging at 20° C. and 0.2 It(A) (500 mA) for 6 hours were repeated 10 times and finally dischargingwas carried out for chemical conversion treatment.

Thereafter, charging in condition of 0.5 It (A) and −dV=5 mV, 30 minutepause, and discharging (20° C.) at 1 It (A) to final voltage of 1V wererepeated and the number of cycles when the discharge capacity became 50%of the initial capacity was defined as the cycle life.

The measurement results are shown in Table 9 and FIG. 10.

TABLE 9 Total ratio of Pr₅Co₁₉ phase, Ce₅Co₁₉ phase, Alloy composition(mol %) and Ce₂Ni₇ phase La Ce Pr Nd Mg Ni Co Al (% by weight) Cyclelife Example 102 14.1 0.0 0.0 4.3 3.3 72.8 2.2 3.3 98 350 Example 10315.2 0.4 0.9 2.0 3.3 72.8 2.2 3.3 98 350 Example 104 14.8 0.7 0.9 2.23.3 72.8 2.2 3.3 98 350 Example 105 14.6 0.9 0.7 2.2 3.5 72.8 2.2 3.3 96340 Example 106 14.8 1.3 0.4 2.0 3.3 72.8 2.2 3.3 95 280 Example 10713.9 2.2 0.2 2.0 3.5 72.8 2.2 3.3 85 150 Example 108 13.9 4.3 0.0 0.03.5 72.8 2.2 3.3 76 50 Example 109 16.3 0.4 0.2 1.3 3.5 72.8 2.2 3.3 88160

As shown in Table 9, with respect to the hydrogen storage alloys of theinvention, it was understood that even if misch metal was used as a rawmaterial, the cycle life could be maintained for a relatively long timeby controlling the Ce content to be 2.2 mol % or lower. Particularly, inthe case of Example 103 to Example 106 in which the Ce content was 1.3mol % or lower and the total ratio of Pr₅Co₁₉ phase, Ce₅Co₁₉ phase, andCe₂Ni₇ phase was 95% by weight or higher were found having extremelyexcellent cycle life, similarly to the hydrogen storage alloy of Example102 containing a high purity material, Nd, in a relatively high amount.

1-4. (canceled)
 5. A hydrogen storage alloy containing two or morecrystal phases having different crystal structures, wherein the two ormore crystal phases are layered in the c-axis direction of the crystalstructures and the hydrogen storage alloy has a composition defined by ageneral formula R1_(d)R2_(e)R4_(f)R5_(g) (wherein R1 is one or more kindelements selected from the group consisting of rare earth metalsincluding Y; R2 is one or more kind elements selected from the groupconsisting of Mg, Ca, Sr, and Ba; R4 is one or more kind elementsselected from the group consisting of Ni, Co, Cr, Fe, Cu, Zn, Si, Sn, V,Nb, Ta, Ti, Zr, and Hf; R5 is one or two elements selected from Mn andAl; and d, e, f, and g satisfy 8≦d≦19; 2≦e≦9; 73≦f≦79; 1≦g≦4; andd+e+f+g=100) and satisfies 3.53≦(B/A)≦3.80 and0.0593(B/A)+1.59≦rA≦0.0063(B/A)+1.81 in the case (B/A) is defined as(f+g)/(d+e) and rA (Å) is defined as the average atomic radius of R1 andR2.
 6. The hydrogen storage alloy according to claim 5, wherein the R1is one or more kind elements R1′ selected from the group consisting ofCe, Pr, Nd, Sm, and Y and La at La/R1′ ratio of 5 or less; R2 is Mg; theR4 is one or two elements selected from Ni and Co; the R5 is Al; and thed, e, f, and g satisfy 16≦d≦19; 2≦e≦5; 73≦f≦78;and2≦g≦4.
 7. The hydrogenstorage alloy according to claim 5 having, as a main produced phase, acrystal phase having Pr₅Co₁₉ type crystal structure or a crystal phasehaving Ce₅Co₁₉ type crystal structure.
 8. A hydrogen storage alloycontaining two or more crystal phases having different crystalstructures, wherein the two or more crystal phases are layered in thec-axis direction of the crystal structures and the hydrogen storagealloy has, as a main produced phase, a crystal phase having Ce₅Co₁₉ typecrystal structure and a composition defined by a general formulaLa_(h)R6_(i)R7_(j)Mg_(k)R8_(m) (wherein R6 is one or more kind elementsselected from the group consisting of rare earth metals including Y andexcluding La; R7 is one or more kind elements selected from the groupconsisting of Zr, Ti, Zn, Sn and V; R8 is one or more kind elementsselected from the group consisting of Ni, Co, Mn, Al, Cu, Fe, Cr, andSi; and h, j, k and m satisfy 0≦j≦0.65; 2≦k≦5.5; 0.70≦h/(h+i)≦0.85;andh+i+j+k+m=100).
 9. A hydrogen storage alloy containing two or morecrystal phases having different crystal structures, wherein the two ormore crystal phases are layered in the c-axis direction of the crystalstructures and the ratio of the crystal phase having CaCu₅ type crystalstructure is 22% by weight or less.
 10. The hydrogen storage alloyaccording to claim 9, wherein the hydrogen equilibrium pressure is 0.07MPa or less.
 11. The hydrogen storage alloy according to claim 9 havinga composition defined by a general formula R1_(n)R2_(p)R4_(q)R5_(r)(wherein R1 is one or more kind elements selected from the groupconsisting of rare earth metals including Y; R2 is one or more kindelements selected from the group consisting of Mg, Ca, Sr, and Ba; R4 isone or more kind elements selected from the group consisting of Ni, Co,Cr, Fe, Cu, Zn, Si, Sn, V, Nb, Ta, Ti, Zr, and Hf; R5 is one or two kindelements selected from Mn and Al; and n, p, q, and r satisfy 16≦n≦23;2≦p≦8; 68.5≦q≦76; 1≦r≦6.5; and n+p+q+=100).
 12. The hydrogen storagealloy according to claim 9, wherein the content of Mn is 5% by weight orless. 13-14. (canceled)
 15. A method for producing a hydrogen storagealloy containing two or more crystal phases having different crystalstructures, wherein the two or more crystal phases, are layered in thec-axis direction of the structures the method comprising a melting stepof heat melting alloy raw materials at prescribed mixing ratio in inertgas atmosphere; a cooling step of rapid solidification the melted alloy;and an annealing step of further annealing the alloy subjected to thecooling step at a temperature that ranges from 860° C. to 1000° C. ininert gas atmosphere in pressurized state.
 16. The method for producingthe hydrogen storage alloy according to claim 15, wherein in theannealing step a pressurizing condition is at a gauge pressure thatranges from 0.2 MPa to 1.0 MPa.